I just permanently put down a book–sadly, no, this is not an “I can in fact quit you” post–because the historian writing it was taking every single period usage of a particular insult as though it was documented fact. He didn’t seem to have realized what the other historians of that period had: namely, that while some behaviors probably were well-supported, a single mention of a political enemy calling someone a witch, a sodomite, a cripple, or some other term assumed by contemporaries to be negative did not mean that they engaged in any particular concrete behavior whatsoever–except to disagree with the political enemy.

We’ve had this in our own lifetimes. Of course we have. People get called gay, Muslims, Communists, terrorists, because the people using those terms think that those are negative things to be, not because they saw someone kissing someone of the same sex or praying to Mecca, much less anything that might in real terms constitute affiliation with any Communist (or communist) or terrorist group. And yet historians! Come on, historians! This is supposed to be your job! This kind of perspective: you’re supposed to know better!

But in terms of writing speculative fiction, I think this kind of culturally pervasive insult can rise above Princess Leia’s “scruffy-looking nerf-herder!” if it’s handled carefully, but only then, and it’s pretty tricky for exactly the reasons the historians had trouble with it. One of the main ways we learn things about characters in a novel is what the other characters say about them, so if Ana says that Bot is a filthy drum-sniffer–what’s a drum-sniffer? why shouldn’t you sniff drums? what do they smell like?–we, the readers, have to find out that there’s some reason not to believe her, or else we do go away with the thought that, well, that Bot, he’s a filthy drum-sniffer. And we have to know whether drum-sniffing is an actual thing that carries with it serious shame, whether it used to but has fallen into slang usage that no longer feels literal (as with “bastard” no longer carrying serious allegations of parental non-marriage), and so on. Whether something is potentially a literal truth and only some people find it insulting is one of the hardest ones to pull off–the speculative world equivalent of allegations that President Obama is a secret Muslim, for example. It’s hard enough to navigate the thickets of “He isn’t, but it wouldn’t be an insult if he was” in this world, where there is an actual President Obama whose external religious behavior can be observed, and where people can look up external definitions of what “Christian” and “Muslim” mean to various parties.

It’s another piece of worldbuilding that can add richness and depth to the culture(s) and personalities you’re building, or it can bog a story down and confuse readers needlessly. If it happens to actual historians, it can certainly happen to fantasy readers. But that’s not a reason not to try, it’s a reason to be careful and run things past test-readers. Like most of the things worldbuilding nerds talk about wanting to see more of, it shouldn’t be required in every story, as a checklist, just as one of the cultural touchstones that can spark implications and ideas.

For something about how complicated an insult can get, see Anti-Judaism, a book about how Jewishness was variously invented by non-Jews and used for their own political purposes. This could include non-Jews who may have never seen a Jew who were accusing each other of being too Jewish because they were on opposite sides of a political issue.

The author whose book I put down touched on that briefly, and the historian seemed to be completely oblivious to the fact that "X said Y was a crypto-Judaizer" had no bearing whatsoever on whether Y had even seen a Jew, had any Jewish ancestors, any Jewish practices, etc.

His main focus was on "homosexuality," using that term, with the implication that it was meaningful in its modern sense in 15th century Spain, and also with several weird and provably wrong assumptions about homosexuality in the first place. But he did manage to get some stuff about Jews, Jewishness, and allegations of same in Spain wrong along the way OH YAY.

I sometimes run into this in fan commentary around perceived inconsistencies in books. When both the Internet and I were much younger, I vividly remember getting into an argument with someone who claimed it a goof that, in the third Harry Potter book, Harry pays a galleon when he gets off the Knight Bus in a hurry "when we know the fare is only a sickle!" This fellow fan was unwilling to accept my alternate reading (it's a telling character moment!), and eventually I learned my lesson about arguing with strangers on the Internet.

Other times I see readers confused when a characters says something which contradict things the readers "know" about the universe or other characters, and I have to restrain myself from suggesting, "Have you considered that they might be lying?" (Or misinformed, or misremembering, or making shit up on the spot, or any of the other reasons people's words are not 100% factual...)

One of the things I like about the idea of the fat fantasy epic which I haven't much seen done is letting the acausal lens of the narrative show us how information takes time to move from one place to another, how it spreads and what effect it has, and I despair a little bit for ever seeing something which really nails it when I see this kind of confusion in fellow readers.

One of the things I have had to let go of is the idea that there will something clear enough that it's clear for every reader. Somebody will misread everything, and they will ascribe some of their misreadings to author error. There is literally nothing to be done about that except deep breathing and acceptance.

But you're describing a problem I think has worse roots, and that is people who cannot cope with even the concept of the unreliable narrator, either as a POV character or in dialog. And that's really frustrating. One of the things I wonder about as a source for it is the people who insist that showing any behavior is endorsing that behavior--if you really have that as the root of your worldview, you cannot possibly teach about unreliable narrators, because that is teaching about lying. (The people who don't allow for any more nuance than that are unlikely to have the rest of the list you provide--misinformed etc.--in their arsenal, I fear.) And that's really scary, because, well, nuance: we need it.